Influence of ligand second coordination sphere effects on the olefin (co)polymerization properties of α-diimine Pd(ii) catalysts

Abstract

A series of α-diimine ligands and their corresponding palladium complexes were synthesized and characterized. These α-diimine ligands possess unique properties, which could lead to metal complexes with very similar electronic and steric environments around a palladium center. For example, the CO stretching frequencies of the palladium carbonyl complexes are very similar. At a remote position on the second coordination sphere of the ligand structure, substituents (Ph, CF₃, NO₂ and OMe) are installed to probe their potential influences on olefin polymerization and copolymerization processes. The properties of these palladium complexes in ethylene polymerization, ethylene/methyl acrylate copolymerization, ethylene/norbornene copolymerization and ethylene/5-norbornene-2-yl acetate copolymerization are investigated in detail. Interestingly, most of the metal complexes behaved very similarly to each other except for the complex bearing a Ph substituent, which showed much better activity and generated polymers with much higher molecular weight.